1. Field of the Invention
The present invention relates to a voltage regulator circuit having a short-circuit protection circuit. More particularly, the present invention relates to a voltage regulator circuit having a short-circuit protection circuit which can limit an output current.
2. Description of the Related Art
A voltage regulator circuit which outputs a predetermined current at a predetermined voltage in response to an input voltage is widely used in electronic devices.
The voltage regulator circuit is the circuit which converts an output current of a driver into a voltage, feeds the voltage back to a differential amplifier, compares the feedback voltage with a reference voltage, and adjusts a drive voltage of the driver based on a comparison result, and then outputs a predetermined current at a predetermined voltage from an output terminal.
In such a voltage regulator circuit, if any reason (for example, solder bridge) causes the short-circuit between the output terminal of the driver and a ground, the feedback voltage becomes 0V. In this case, the differential amplifier continues the action to increase the drive voltage of the driver. If the differential amplifier continues this action, there may be a case that the circuit is broken by Joule heat generation.
For this reason, typically, the voltage regulator circuit has a short-circuit protection circuit for stopping the operation of the differential amplifier if a trouble such as an earth fault of the output terminal or the like occurs. Here, the earth fault of the output terminal is the short-circuit to the ground.
In conjunction with the voltage regulator circuit having the short-circuit protection circuit, a conventional technique of a regulator is disclosed in Japanese Laid Open Patent Application JP 2003-173211A.
FIG. 1 is a circuit diagram showing the configuration of the voltage regulator circuit having the short-circuit protection circuit according to the conventional technique. This voltage regulator circuit 101 includes a regulator circuit 102 and a short-circuit protection circuit 103. The regulator circuit 102 includes a reference voltage source 111, an amplifier 112, a MOS transistor 113, a resistance 114 and a resistance 115. The short-circuit protection circuit 103 includes an amplifier 122, a MOS transistor 123 and a resistance 124.
This voltage regulator circuit 101 divides the output voltage from the MOS transistor 113 and feeds a divided voltage Vb back to the amplifier 112. The amplifier 112 controls a gate voltage of the MOS transistor 113 such that a reference voltage VREF supplied from the reference voltage source 111 coincides with the feedback voltage Vb.
The output voltage VIN from the amplifier 112 is supplied to both of the MOS transistor 113 and the MOS transistor 123 as a gate voltage Vg.
A current value IL based on the output current of the MOS transistor 123 is voltage-converted into a voltage Va based on the resistance 124 and supplied to the amplifier 122. Also, the feedback voltage Vb that is fed through a voltage division resistor 114 to the amplifier 112 is also supplied to the amplifier 122. In the voltage regulator circuit 101 having the above-mentioned configuration, an output current IOUT of the MOS transistor 113 when the short-circuit protection circuit 102 is operated is represented by an equation (1). Incidentally, in the equation (1), μ indicates an electron mobility in the MOS transistors 113, 123. COX indicates a fixed capacity of the gate insulating films in the MOS transistors 113, 123. W1 and L1 indicate a channel width and a channel length of the MOS transistors 113. W2 and L2 indicate a channel width and a channel length of the MOS transistors 123. Ra1 indicates a resistance value of the resistance 124.
[Equation (1)]
                                                                        I                OUT                            =                                                                    W                    1                                                        L                    1                                                  ·                k                ·                                                                            V                      b                                        ·                                          L                      2                                                                                                  R                      a1                                        ·                                          W                      2                                        ·                    k                                                                                                                          k              =                              μ                ·                                  C                                      0                    ⁢                    X                                                  ·                                  1                  2                                                                                        (        1        )            
As can be seen from the equation (1), in the voltage regulator circuit 101 having the above-mentioned configuration, the output current IOUT from the MOS transistor 113 is a function inversely proportional to the resistance value Ra1 of the resistance 124 in the short-circuit protection circuit 103.
In a manufacturing process of a semiconductor device, an absolute value of a circuit element is largely varied. For example, since a variation in a patterning process and a variation in a diffusing process are overlapped, it is difficult to attain the values in a design. The actually produced circuit element contains an error of about ±30%.
In the voltage regulator circuit 101 having the conventional short-circuit protection circuit 103, the output current IOUT from the MOS transistor 113 serving as the driver is represented as the function inversely proportional to the resistance value Ra1 of the resistance 124. It has now been discovered that the variation in the resistance value Ra1 directly corresponds to the variation in the output current IOUT.
FIG. 2 is a graph showing the output property of the voltage regulator circuit having the short-circuit protection circuit according to the conventional technique. Incidentally, a curve “a” in the graph indicates a designed standard value of a short-circuit current, a curve “b” in the graph indicates the minimum condition of the short-circuit current, and a curve “c” in the graph indicates the maximum condition of the short-circuit current.
As illustrated, in the voltage regulator circuit 101 having the short-circuit protection circuit 103 according to the conventional technique, even if the drive currents of the driver are equal, there is a large variation in the values of the output currents IOUT actually outputted from the MOS transistor 113.
In order to remove such variation, the trimming must be performed to the resistance value Ra1 to adjust the resistor value. This causes the problems of the increase in the number of the processes in the manufacturing process of the voltage regulator circuit and the increase in the manufacturing cost.
Incidentally, the invention, which is disclosed in the JP 2003-173211A and is the short-circuit protection circuit having a blocking property, similarly to the above-mentioned case, the variation in the resistance value of the resistance results in the variation in the output current.
In this way, the voltage regulator circuit having the conventional short-circuit protection circuit has a problem that it is difficult to obtain the desirable circuit property because it receives the influence of the manufacture variation in the resistance value of the resistance.